Just as Star Wars: The Old Republic, a massive multiplayer online game of one hugely popular sci-fi franchise launches, another hits the 'net. And this one is free to play.

Doctor Who: Worlds in Time is BBC Worldwide's first MMO featuring the globally successful Doctor and, rather than forcing players to download a client, it is played entirely in browser. Designed by online games specialist Three Rings, it is freemium, so credits can be bought to speed up the acquisition of certain power-ups and items, but it is possible to play the game to its fullest without having to spend a single penny of real money.

Pocket-lint was given a full demo of the game, which is available to play from today in an open preview build, by assistant producer, Games, BBC Worldwide, Ben Milsom and Robert Nashak, executive vice president, BBC Worldwide Digital Entertainment and Games. And we must say that we're impressed... Very impressed, indeed.

For starters, the premise is excellent; rather than allow you to play the Doctor himself, or an alternative Timelord (which would break continuity with the show), you get to design and take on the role of a brand new Doctor Who assistant. Your character is fully customisable (aesthetically) and you can even choose from multiple races plucked from the show's vast history. You then get to play the adventures (or "interventions") without the direct help of the Doctor himself (although the Matt Smith version of the Timelord does give you guidance along the way).

Your goal, as this character, is to save worlds which are under peril through, mainly, a wide variety of puzzles. At present, there are 10 big worlds, including Skaro, Messaline and New New York, which you need to save from 10 iconic enemies from the Doctor's past and, although the game is heavily influenced by the more recent series, these can range across the entire show's (almost) 50-year history - enough to get any true fan excited, young and old.

The game is, if you want a reference point, similar to Puzzle Quest in some ways, although you must explore in order to find out what you have to do, and team up with other players to solve some of the mini-games and perform functions, such as open locked doors, etc. And the puzzles come in a variety of forms, from match-three games to Tetris-style block building and more.

It's not a role-playing game in the traditional sense, as you don't get to improve your character's talents or choose classes - everybody starts on an even keel. However, each player is given their own room on the Tardis to adorn with trophies and collectables from the game, and, more importantly, a "Gadget".

The Gadget is, basically, the player's Sonic Screwdriver, and it is here that each character can build up a different skill set, and therefore become unique. You can add abilities in the form of power-ups to the device, and then use it in different ways in the game.

For example, if you add a lockpick-style power-up to your Gadget, you will be able to crack through the security on doors more speedily than without. Plus, if you team up with players who also have similar abilities on their Gadgets, you can combine to quicken actions further. That also means that you can team up with players with different abilities on their Gadgets, and create a party with a range of strengths.

Items and customisations can be found throughout the levels or bought with the in-game currency Chronons. These are rewarded for puzzle-solving, but also build up automatically on a day to day basis, and can be spent on new outfits for your avatar or power-ups for the Gadget. It is these that can be bought with real money through a micro-transaction model. However, as previously stated, it's possible to still accumulate the same additional features through persistence, it just takes longer.

As the current version of the game is an open preview build, there are some features that will be added when Doctor Who: Worlds in Time hits full commercial release around March 2012. One of these will be the option to start and join guilds, thereby offering a traditional option familiar to hardcore gamers.

As Robert Nashak told us, there's not really an age limit on the ideal audience for the game. BBC Worldwide fully expects it to appeal to Doctor Who fans from 13-years and up, including those who's fondest memories of the TV show could've come from Jon Pertwee, Tom Baker and, even, William Hartnell's days. The puzzle mechanic is easy to understand, and there's plenty of secrets and surprises for the die-hard Whovians out there.

The BBC is also making a long-term commitment to Doctor Who: Worlds in Time and promises that there will be perpetual extra content and support. New worlds will be added, and new storylines that match those that are seen on TV as and when the new series starts. Plus, Nashak hinted, there will be major developments surrounding the 50th Anniversary of Doctor Who next year. We can't wait.

You can sign up to the open preview of Doctor Who: Worlds in Time at DoctorWhoWIT.com. We'll see you there.

What do you think of the game and graphical style? Has the BBC got it right with the freemium model? Let us know in the comments below...